This invention solves a need for a very thin foil of high purity material that is highly reactive to an air atmosphere. Specifically, a lithium sample of thin cross section of less than 0.010 inch and of very high purity was needed for use with neutron bombardment to examine the presence or release of other materials, including titanium. As high purity was needed in the foil, it could not be rolled as that would leave an oil film on the surfaces. Moreover, as lithium is highly reactive in the presence of the air, special care was needed to maintain purity of the fabricated foil. Sodium is another material that is very useful in thin foil form, but it too is highly reactive in air and is most difficult to form.
A likely method of forming such a sample foil might include extruding it; however the formation of the slot in the extension die was particularly difficult. The slot opening, approximately 0.75-1.25 inches long and less than 0.01 inch wide, would thus be formed through a relatively thick wall of a structural material comprising the die. This invention utilizes an electric discharge machine to form the slot.
In an electric discharge machine, the work piece is submerged in a bath of oil and an electrode (typically at ground potential) is moved against the work piece (typically held at a large potential, for example, 250 volts relative to the ground). Arcing under the oil thereby occurs as the electrode approaches and just touches the work piece, and the arcing erodes away or "cuts" the work piece. The work piece must be flushed with the oil during the cutting operation. It is possible to use a fine wire as the electrode for simply cutting the work piece in two, or by means of elaborate camming and guiding means for cutting exotic shapes in the work piece. If the "cut" desired is a hole or slot, the shape of the electrode can determine the shape of the cut by simply moving the electrode into the work piece. If the electrode is large in cross section, it generally is of hollow construction and the oil can be pumped through it and against the work piece. Prior to this invention, the electric discharge machine did not function properly in cutting a slot with a narrow or thin electrode.